A tire air pressure monitoring system (TPMS) for monitoring a tire air pressures of tires of a vehicle is known (for example, Japanese Laid-open Patent Publication No. 2009-202675).
However, according to the conventional TPMS as described above, various notifications regarding the tire air pressure cannot be output in a user-friendly manner. For example, when a notification indicating a reduced state of the tire air pressure is output without any previous sign, the notification becomes an unexpected alarm for a user, which causes the user to have difficulty with how to respond to it. Moreover, when the user mistakes such a notification for a puncture alarm (tire deflation warning), there is a probability that the notification unnecessarily causes anxiety to the user.